Indiana has nonpartisan registration. There is no option to affiliate with a political party on the voter registration form.
Details of voting in congressional and state primary elections:
Affiliation with a party is not required to vote in primaries. However, voters can only choose the primary ballot of the party who recieved a majority of their votes in the previous general election, and voter records are kept as public information.
If a voter did not vote in the last general election, they must "intend to vote for the majority of the nominees on their desired party's ballot."
Voters can be challenged by another eligible voter on suspicion of perjury.
This system is an attempt to get voters to vote along party lines, but is not easily enforceable.